Roughly eighty percent of energy consumption in the United States is derived from fossil fuels. Conversion of these raw fuels to process streams that are more readily usable has long been possible, however, in recent decades due to more stringent environmental regulations, there has been increasing emphasis on also minimizing the presence of sulfur-containing compounds in the process streams. These sulfur compounds are present in fossil fuels, whether the fuel is liquid (crude oil), solid (coal), or gas (natural gas). Methods for removing sulfur, or converting it to more readily-processable forms are so important that it is sometimes necessary to operate entire refinery units specifically devoted to that purpose. For example, fluid catalytic cracking units, which process downstream derivative streams of crude oil, often pretreat their feed in hydrotreating units to convert sulfur-containing compounds to materials that do not boil in the gasoline range. In the treatment of natural gas or coal-derived syngas, hydrogen sulfide (H2S) is found. Hydrogen sulfide not only presents environmental concerns, but is poisonous to catalysts and corrosive to metals, and therefore needs to be removed from the process streams. This is often achieved using metal oxides as reactive adsorbents, but such materials often perform poorly when subjected to repeated cycles of sulfidation and re-oxidation because of complex structural and chemical changes. Cyclic use of the adsorbent degrades its performance for a variety of reasons, including grain growth-led reduction in specific surface area (sintering), sorbent underutilization due to diffusion-limited gas solid reactions, mechanical spalling (adsorbent breakup) due to reaction and heat induced volumetric changes, and formation of non-regenerable, thermodynamically stable side products.
The bulk of research in this area has mainly focused on modifying sorbent chemical composition. However, because the overall gas-solid reaction is often controlled by diffusion, tuning chemical properties alone limits the number of possible solutions. Conventional pellet-based sorbent designs are transport limited in sulfur uptake, which exposes the outermost layers to disproportionately longer durations, in comparison to the pellet interior, leading to sorbent fragmentation. Nanosized H2S adsorbents, with their large specific surface areas and short diffusion lengths, would seem to be more appropriate when using such an approach. However, in addition to size, the choice of sorbent morphology is also important. Particulate-based adsorbents tend to aggregate and sinter together under high temperature cycling, causing diffusion barriers similar to bulk sorbents. Therefore, a sorbent morphology that promotes faster overall kinetics while simultaneously preventing progressive material underutilization is highly desirable.
Work has been ongoing to develop improved adsorbents. U.S. Pat. No. 5,271,907 discloses high temperature desulfurization of coal-derived gases using regenerable sorbents. U.S. Pat. No. 5,703,003 discloses durable, regenerable sorbent pellets for removal of hydrogen sulfide from coal gas. U.S. Pat. No. 5,741,469 discloses a dry, regenerable solid oxide process for converting SOx in flue gas streams to elemental sulfur without using a Claus unit. Nevertheless, a continuing need exists for reactive adsorbents with high levels of reactivity that are capable of withstanding multiple cycles of sulfidation/regeneration while maintaining activity. It has unexpectedly been found that zinc titanate based adsorbents with a nanofibrous morphology exhibit high sulfur removal capacity over multiple regeneration cycles, accompanied by rapid reaction rates. The nanofibrous morphology overcomes transport-related limitations, and enables complete material utilization by promoting reaction-controlled kinetics. In-situ nanoscale stabilization of the adsorbent's ZnS wurtzite phase and the growth of rate-enhancing hierarchical structures further facilitate sorbent regeneration.